The phrase "for sure" is used to express certainty or confirmation about something. When someone says "for sure," they are emphasizing that they are confident about the information or opinion they are presenting. It reinforces the idea that there is no doubt or uncertainty regarding what is being discussed.
The phrase "for sure" derives from the combination of the preposition "for," which comes from the Old English "for," meaning "because of" or "on account of," and "sure," which has roots in the Latin "securus," meaning "safe" or "secure." Together, they form an expression that conveys a sense of reassurance and certainty.